A new theory model based on the local composition concept (TNRF-modified NRTL (TNRF-mNRTL) model) was developed to express the short-range contribution of the excess Gibbs energy for the solutions containing polymer and electrolyte. An equation represented the activity coefficient of solvent was derived from the proposed excess Gibbs energy equation. The short-range contribution of interaction along with the long-range contribution of interaction and configurational entropy of mixing were used to correlate the activity coefficient of ternary polymer + electrolyte + water systems and also binary polymer + water and electrolyte + water systems. The long-range interaction and configurational entropy have been given by the Pitzer-Debye-Hückel equation and the Flory-Huggins relation, respectively. The performance of the proposed model in fitting the solvent activity of ternary polymer + electrolyte + water solutions has been compared with that obtained from the ternary NRTL, ternary Wilson, ternary modified NRTL and ternary modified Wilson models. Results comparison was demonstrated the validity of the proposed model for solvent activity of polymer + electrolyte + water solutions.